The present invention is about one kind of integrated circuits and its manufacturing processes. More particularly, the invention provides a method for manufacturing a mask read only memory (“ROM”) having reduced critical size features that are substantially free from undesirable punch through effects. But it would be recognized that the invention has a much broader range of applicability. For example, the invention can be applied to embedded ROM designs and others.
Integrated circuits have evolved from a handful of interconnected devices fabricated on a single chip of silicon to millions of devices. Conventional integrated circuits provide performance and complexity far beyond what was originally imagined. In order to achieve improvements in complexity and circuit density (i.e., the number of devices capable of being packed onto a given chip area), the size of the smallest device feature, also known as the device “geometry”, has become smaller with each generation of integrated circuits.
Increasing circuit density has not only improved the complexity and performance of integrated circuits but has also provided lower cost parts to the consumer. An integrated circuit or chip fabrication facility can cost hundreds of millions, or even billions, of dollars. Each fabrication facility will have a certain throughput of wafers, and each wafer will have a certain number of integrated circuits on it. Therefore, by making the individual devices of an integrated circuit smaller, more devices may be fabricated on each wafer, thus increasing the output of the fabrication facility. Making devices smaller is very challenging, as each process used in integrated fabrication has a limit. That is to say, a given process typically only works down to a certain feature size, and then either the process or the device layout needs to be changed.
Many types of integrated circuit devices including memory devices, application specific integrated circuit devices, microprocessor device, and others exist. Memory devices include read only memories such as ROMs. ROMs include mask ROMs and field oxidation ROMs. As device sizes become smaller, various limitations exist with mask ROMs. For example, mask ROMs have a punch through problem, where a cell improperly provides an incorrect state of the cell upon a reading operation. That is, the cell outputs a “1” rather than “0” state, which yields an improper output. As device sizes become smaller, such problem becomes worse. A threshold voltage of the cell drops and causes improper reading of the cell. These and other limitations exist with conventional ROM integrated circuit devices.
From the above, it is seen that an improved technique for processing semiconductor devices is desired.